Abstract
The proton-bound dimer of hydrogen sulfate and formate is an archetypal structure for ionic hydrogen-bonding complexes that contribute to biogenic aerosol nucleation. Of central importance for the structure and properties of this complex is the location of the bridging proton connecting the two conjugate base moieties. The potential energy surface for bridging proton translocation features two local minima, with the proton localized at either the formate or hydrogen sulfate moiety. However, electronic structure methods reveal a shallow potential energy surface governing proton translocation, with a barrier on the order of the zero-point energy. This shallow potential complicates structural assignment and necessitates a consideration of nuclear quantum effects. In this work, we probe the structure of this complex and its isotopologues, utilizing infrared (IR) action spectroscopy of ions captured in helium nanodroplets. The IR spectra indicate a structure in which a proton is shared between the hydrogen sulfate and formate moieties, HSO4–···H+···–OOCH. However, because of the nuclear quantum effects and vibrational anharmonicities associated with the shallow potential for proton translocation, the extent of proton displacement from the formate moiety remains unclear, requiring further experiments or more advanced theoretical treatments for additional insight.
Highlights
The ionic hydrogen bond is a prominent structural motif found in diverse chemical systems
The temperature dependence of the negative-ion photoelectron spectra was attributed to the shifts in the equilibrium population of these two structures resulting from the difference in free energy. We study this complex and its isotopologues by IR action spectroscopy of ions captured in helium nanodroplets
As noted in previous studies,[27,67] the employed action spectroscopy technique exhibits a nonlinear dependence of line intensity on transition strength at a given FEL macropulse energy, and the relative intensity of strong or weak lines may be exaggerated or diminished, respectively
Summary
The ionic hydrogen bond is a prominent structural motif found in diverse chemical systems. The mechanical and functional significance of this linkage arises from its unique properties, including strong interaction energies on the order of 20−150 kJ mol−1 and the capability to participate in proton transfer reactions.[1−3] Of particular interest is the complex formed when two anionic Brønsted conjugate bases are bridged by a single proton, yielding a singly charged anionic moiety, here denoted as AHA− Such structures are found, for example, in proteins,[3−6] aerosol prenucleation clusters,[7−13] and ionic liquids.[14−16] The precise location of the bridging proton in these complexes whether the proton is shared (A−···H+···A−) or localized on a single residue (AH···A−) has been the subject of extensive research in systems ranging from deprotonated water clusters to proteins.[2,3,5,17−20] These inquiries are motivated by the consequences of proton location for both overall structure and molecular properties such as effective pKa.[2,21]. Cryogenic ion IR spectroscopy, in which ions are buffer-gas cooled to temperatures of 10−100 K prior to spectroscopic interrogation, has proven effective.[12,22] This technique has been applied to the study of deprotonated water clusters,[17,19] model carboxylate protonbound dimers,[26−28] and putative aerosol nucleation clusters.[12,13] The structure of AHA− systems has been investigated utilizing photoelectron spectroscopy[9−11,29] and room-temperature IR action spectroscopy.[24,25]
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